1. Field of the Invention
Example embodiments of present invention generally relate to a power control adapted for use in a direct sequence code division multiple access (DS-CDMA) system. More particularly, example embodiments of the present invention relates to a signal-to-interference ratio (SIR) estimation adapted for use in a closed-loop power control.
2. Description of the Related Art
In general, a direct sequence code division multiple access (DS-CDMA) wireless protocol specifies interfaces in wireless communication systems such as a code division multiple access (CDMA) system and a wide-band code division multiple access (W-CDMA) system. In these systems, an accurate power control may be necessary to effectively utilize frequency resources that are commonly shared by a mobile station and a base station according to spread-spectrum characteristics of the wireless communication system.
A system capability, possibly larger in a spread spectrum type system, may be increased when the power is optimally controlled, because the system capability of the CDMA system or the W-CDMA system may be highly correlated with an amount of interference signals received on the system, which may be determined by an intensity of a signal transmitted to/from each mobile device.
The W-CDMA standard is embodied in the 3rd Generation Partnership Project (3GPP) standard. For example, in the first or second generation system, a digital cellular or a personal communication system, data traffic on a reverse link may be substantially equal to data traffic on a forward link, because the first and second generation communication systems were designed to mainly carry speech and low bit-rate data.
However, the third generation communication systems such as W-CDMA are designed to carry not only speech but also packet data having a higher data rate. The third generation communication systems are also adapted to carry higher data-rate wireless Internet services.
One example characteristic of the wireless Internet service is that a forward link may provide a service of downloading a large capacity picture movie, voice data, picture data, and program data. The forward link may have more traffic than a reverse link. Therefore, the traffic on the two links may be asymmetric. In the W-CDMA system, capacity limitation of a mobile device may be more likely to be generated at the forward link rather than the reverse link.
To overcome the capacity limitation, a closed loop power control may be employed in the forward link of the W-CDMA system in the 3GPP standard to efficiently manage resources of the forward link.
The forward closed-loop power control, and those especially based on a signal-to-interference ratio (SIR), may adjust a transmit power of a base station on a forward link based on a received power estimate at a mobile device such that the SIR of each respective mobile device may be constantly maintained. In other words, the transmit power of the base station may be adjusted according to the state of the mobile device, for example by taking into account a near-far effect. Therefore, when the mobile device is not operated (e.g., the mobile device is not connected) but relatively close to a base station or has a relatively low levels of multi-path fading, shadowing, interference from other base stations, etc., transmit power may be reduced. On the other hand, when the mobile device is relatively farther away from the base station, which may create higher bit error rate or a signal reception by the mobile device is poor, the transmit power is increased.
In the closed-loop power control based on the SIR estimation, a reference SIR is compared with an SIR estimate per a predetermined power control period (e.g., 1/1500 sec, in the 3GPP). Therefore, the accuracy of the SIR estimation may greatly affect the stability and accuracy of the transmit power control.
A conventional SIR estimation method in accordance with an NTT DoCoMo wireless protocol is described in “SIR-Based Transmit Power Control of Reverse Link for Coherent DS-CDMA Mobile Radio,” IEICE Transactions on Communications, July 1998. According to the above disclosed SIR estimation method, after a tentative decision based on a channel estimation using a pilot symbol, the SIR is estimated using an average power per slot and a mean squared error of a dedicated physical channel (DPCH). The tentative decision herein is made such that a hard decision is made for a phase of a channel compensated received signal to return a phase of a non-channel compensated signal. The tentative decision is used for signal power estimation per slot, using not only a pilot symbol of which a phase is known, but also a random data symbol.
A transmitting signal including the DPCH signal and a common physical channel (CPICH) signal that are coded and multiplexed to be transmitted in parallel may be represented by equation 1, and a received signal passing through a fading channel and Additive White Gaussian Noise (AWGN) may be represented by the following equation 2:
                              S          ⁡                      (            t            )                          =                              ∑                          i              =                              -                ∞                                      ∞                    ⁢                      [                          {                                                                    E                                          c                      ,                      cp                                                                      ·                                  G                  p                                ·                                  (                                                                                    C                        I                                            ⁡                                              (                        t                        )                                                              +                                          j                      ⁢                                                                                          ⁢                                                                                                    C                            Q                                                    ⁡                                                      (                            t                            )                                                                          ·                                                  (                                                                                    W                                                              CP                                ⁢                                                                                                    i                                                                                                  ⁢                                N1                                                            I                                                        +                                                          W                                                              CP                                ⁢                                                                                                    i                                                                                                  ⁢                                N1                                                            Q                                                                                )                                                                                      +                                                                                            E                                                      c                            ,                            dp                                                                                              ·                                              G                        p                                            ·                                              (                                                                                                            D                              I                                                        ⁡                                                          (                              t                              )                                                                                +                                                      j                            ⁢                                                                                                                  ⁢                                                                                                                            D                                  Q                                                                ⁡                                                                  (                                  t                                  )                                                                                            ·                                                              (                                                                                                      W                                                                          DP                                      ⁢                                                                                                                      i                                                                                                                    ⁢                                      N2                                                                        I                                                                    +                                                                      W                                                                          DP                                      ⁢                                                                                                                      i                                                                                                                    ⁢                                      N2                                                                        Q                                                                                                  )                                                                                                                                    }                                            ·                                              (                                                                              S                                                                                                                          i                                                                                            ⁢                              M                                                        I                                                    +                                                      S                                                                                                                          i                                                                                            ⁢                              M                                                        Q                                                                          )                                                                                                                                                    [                  Equation          ⁢                                          ⁢          1                ]                                                      r            ~                    ⁡                      (            t            )                          =                              ∑                          i              =                              -                ∞                                      ∞                    ⁢                      [                                          a                ⁡                                  (                  t                  )                                            ·                                                                 [                                                                                    {                                                                                                            E                                                              c                                ,                                cp                                                                                                              ·                                                      G                            p                                                    ·                                                      (                                                                                                                            C                                  I                                                                ⁡                                                                  (                                  t                                  )                                                                                            +                                                              j                                ⁢                                                                                                                                  ⁢                                                                                                                                            C                                      Q                                                                        ⁡                                                                          (                                      t                                      )                                                                                                        ·                                                                      (                                                                                                                  W                                                                                  CP                                          ⁢                                                                                                                                  i                                                                                                                                ⁢                                          N1                                                                                I                                                                            +                                                                              W                                                                                  CP                                          ⁢                                                                                                                                  i                                                                                                                                ⁢                                          N1                                                                                Q                                                                                                              )                                                                                                                              +                                                                                                                                    E                                                                          c                                      ,                                      dp                                                                                                                                      ·                                                                  G                                  p                                                                ·                                                                  (                                                                                                                                                    D                                        I                                                                            ⁡                                                                              (                                        t                                        )                                                                                                              +                                                                          j                                      ⁢                                                                                                                                                          ⁢                                                                                                                        D                                          Q                                                                                ⁡                                                                                  (                                          t                                          )                                                                                                                                                                                      )                                                                ·                                                                  (                                                                                                            W                                                                              DP                                        ⁢                                                                                                                            i                                                                                                                          ⁢                                        N2                                                                            I                                                                        +                                                                          W                                                                              DP                                        ⁢                                                                                                                            i                                                                                                                          ⁢                                        N2                                                                            Q                                                                                                        )                                                                                                                      }                                                    ·                                                      (                                                                                          S                                                                                                                                          i                                                                                                        ⁢                                  M                                                                I                                                            +                                                              S                                                                                                                                          i                                                                                                        ⁢                                  M                                                                Q                                                                                      )                                                                          ]                                            ·                                              ⅇ                                                  jθ                          ⁡                                                      (                            t                            )                                                                                                                +                                                                  n                        ^                                            ⁢                                                                      i                                                                                                      ]                                                                                        [                  Equation          ⁢                                          ⁢          2                ]            Herein,    Ec,cp, Ec,dp: average chip energy of the CPICH and the DPCH    N1, N2: spreading factor (SF) of the CPICH and the DPCH    M: length of a scrambling code    Gp: channel gain of the CPICH    WICP|i|N1+WQCP|i|N1, WIDP|i|N2+WQDP|i|N2: orthogonal variable spreading factor (OVSF) of the CPICH and DPCH    SI|i|M+SQ|i|M: scrambling code    {circumflex over (n)}|i|: AWGN    α(t), θ(t): amplitude and phase of the received signal due to the fading
After performing descrambling and dispreading processes, respective signals of the DPCH and CPICH that are on a chip basis are converted to signals ZDPCH and ZCPICH that are on a symbol basis, which are represented by equations 3 and 4:
                              Z          DPCH                =                                            1                              N                2                                      ⁢                                          ∑                                  k                  =                  0                                                                      N                    2                                    -                  1                                            ⁢                                                          ⁢                              [                                                      a                    ⁡                                          (                      k                      )                                                        ·                                      [                                                                                            E                                                      c                            ,                            cp                                                                                              ·                                              (                                                                                                            D                              I                                                        ⁡                                                          (                              k                              )                                                                                +                                                      j                            ⁢                                                                                                                  ⁢                                                                                          D                                Q                                                            ⁡                                                              (                                k                                )                                                                                                                                    )                                                              ]                                    ·                                      ⅇ                                          jθ                      ⁡                                              (                        k                        )                                                                                            ]                                              +                                    1                              N                2                                      ⁢                                          ∑                                  k                  =                  0                                                                      N                    2                                    -                  1                                            ⁢                                                          ⁢                              W                                  dp                  ⁢                                                          k                                                                                                                              [                  Equation          ⁢                                          ⁢          3                ]                                          Z          CPICH                =                                            1                              N                1                                      ⁢                                          ∑                                  k                  =                  0                                                                      N                    1                                    -                  1                                            ⁢                                                          ⁢                              [                                                      a                    ⁡                                          (                      k                      )                                                        ·                                      [                                                                                            E                                                      c                            ,                            cp                                                                                              ·                                              (                                                                                                            C                              I                                                        ⁡                                                          (                              k                              )                                                                                +                                                      j                            ⁢                                                                                                                  ⁢                                                                                          C                                Q                                                            ⁡                                                              (                                k                                )                                                                                                                                    )                                                              ]                                    ·                                      ⅇ                                          jθ                      ⁡                                              (                        k                        )                                                                                            ]                                              +                                    1                              N                1                                      ⁢                                          ∑                                  k                  =                  0                                                                      N                    1                                    -                  1                                            ⁢                                                          ⁢                              W                                  cp                  ⁢                                                          k                                                                                                                              [                  Equation          ⁢                                          ⁢          4                ]            Herein,WDP|i|=(nI|i|+nQ|i|)·(SI|i|M+SQ|i|M)*·(WIDP|i|N2+WQDP|i|N2)*WCP|i|=(nI|i|+nQ|i|)·(SI|i|M+SQ|i|M)*·(WIDP|i|N1+WQDP|i|N1)*
After a tentative decision is made for the DPCH symbol, the DPCH symbol is rewritten as equation 5 and the signal power at the time of the SIR estimation is represented as the average power of the DPCH symbol per slot, for which the tentative decision is made as in equation 6:
                                          Z            )                    DPCH                =                                                            E                                  c                  ,                  dp                                                      ·                          1                              N                2                                              +                      j            ⁡                          [                                                                                          E                                              c                        ,                        dp                                                                              ·                                      1                                          N                      2                                                                      ⁢                                                      ∑                                          k                      =                      0                                                                                      N                        2                                            -                      1                                                        ⁢                                                                          ⁢                                      [                                                                                            a                          ⁡                                                      (                            k                            )                                                                          ·                        sin                                            ⁢                                                                                          ⁢                                              θ                        ⁡                                                  (                          k                          )                                                                                      ]                                                              ]                                +                                    1                                                N                  2                                ⁢                                                                                        ⁢                                          ∑                                  k                  =                  0                                                                      N                    2                                    -                  1                                            ⁢                                                          ⁢                              Z                                                    k                                                  I                                              +                      Z                                        k                                      Q                                              [                  Equation          ⁢                                          ⁢          5                ]                                                      E            ⁡                          [                                                Z                  )                                DPCH                            ]                                slot          2                =                                            E                              c                ,                dp                                      ·                          [                                                                    (                                                                  1                                                  S                          2                                                                    ⁢                                              1                                                  N                          2                                                                    ⁢                                                                        ∑                                                      s                            =                            0                                                                                                              S                              2                                                        -                            1                                                                          ⁢                                                                                                  ⁢                                                                              ∑                                                          k                              =                              0                                                                                                                      N                                2                                                            -                              1                                                                                ⁢                                                                                                          ⁢                                                      [                                                                                                                            a                                  ⁡                                                                      (                                                                          s                                      ,                                      k                                                                        )                                                                                                  ·                                cos                                                            ⁢                                                                                                                          ⁢                                                              θ                                ⁡                                                                  (                                                                      s                                    ,                                    k                                                                    )                                                                                                                      ]                                                                                                                )                                    2                                +                                                      (                                                                  1                                                  S                          2                                                                    ⁢                                              1                                                  N                          2                                                                    ⁢                                                                        ∑                                                      s                            =                            0                                                                                                              S                              2                                                        -                            1                                                                          ⁢                                                                                                  ⁢                                                                              ∑                                                          k                              =                              0                                                                                                                      N                                2                                                            -                              1                                                                                ⁢                                                                                                          ⁢                                                      [                                                          a                              ⁢                                                                                                (                                                                      s                                    ,                                    k                                                                    )                                                                ·                                cos                                                            ⁢                                                                                                                          ⁢                                                              θ                                ⁡                                                                  (                                                                      s                                    ,                                    k                                                                    )                                                                                                                      ]                                                                                                                )                                    2                                            ]                                =                                    E                              c                ,                dp                                      ·                          K              DPCH                                                          [                  Equation          ⁢                                          ⁢          6                ]            Herein,Z |k|=ZI|k|+ZQ|k|=w|k|·(DI(K)+jDQ(K))*    S2: a number of DPCH symbols per slot    average AWGN per slot:
            1              S        2              ⁢          1              N        2              ⁢                  ∑                  s          =          0                                      S            2                    -          1                    ⁢                          ⁢                        ∑                      k            =            0                                              N              2                        -            1                          ⁢                  (                                    Z                                                                s                  ,                  k                                                            I                        +                          j              ⁢                                                          ⁢                              Z                                                                        s                    ,                    k                                                                    Q                                              )                      ≅  0
It can be determined from equation 6 that the signal power is affected by the fading with a signal power attenuation ratio KDPCH. The signal power affected by the fading with the signal power attenuation ratio KDPCH can be experimentally identified.
FIG. 1 is a graph illustrating a power attenuation of a signal on a dedicated physical channel (DPCH) that undergoes fading.
Referring to FIG. 1, as the fading becomes faster, the power attenuation of a signal becomes greater. Compared with an ideal case where there is no fading (101), the signal power attenuation is generated when a mobile device is moving at a speed of about 120 km/h (103) and when a mobile device is moving at a speed of about 250 km/h (105).
When energy per symbol to interference power density ratio (Es/No) is low, the AWGN may not approach zero, which may generate an error between an actual signal power attenuation and a calculated value by the signal power attenuation ratio KDPCH. When the energy per symbol to interference power density ratio (Es/No) is relatively high, the AWGN may approach zero so that the signal power attenuation is nearly equal to the signal power attenuation ratio KDPCH.
Therefore, in the conventional SIR estimation method, the SIR estimation may not be sufficiently accurate due to the signal power attenuation of a received signal in the fading environment, especially in a fast fading environment. Accordingly, accurate power control depending on a state of a wireless mobile channel may be difficult.